Question

1.) Explain why gravity does positive work on an elevator as the elevator descends from the top floor to the ground floor of a building.

2.) A skateboarder coasting down a ramp. In everyday terms, she speeds up as she coasts down. Describe the same motion in terms of her kinetic energy (KE), potential energy (PE), and total mechanical energy (ME): how does each quantity change, if at all?

3.) At a certain instant, an object's KE is 500 J and its PE is 100 J. A short time later, its KE is 400 J and its PE is 300 J. Explain why the net work done on the object during this interval is negative, but the total amount of work done on it by non-conservative forces is positive.

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1) Work is equal to force times distance. If the direction of motion is considered positive, then the elevator travels a positive distance and the force is positive (since gravity is acting in the same direction that the elevator is moving). If the upward direction is positive then...

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